P2 A networking program usually has two programs, each running on a different host, communica9ng with each other. The program that ini9ates the

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1 P2 A networking program usually has two programs, each running on a different host, communica9ng with each other. The program that ini9ates the communica9on is the client. Typically, the client program requests and receives services from the server program.

2 P3 - In the OSI protocol model, physical communica9on between peers takes place only in the lowest layer, not in every layer

3 P4 Message and byte streams are different. In a message stream, the network keeps track of message boundaries. In a byte stream, it does not. For example, suppose a process writes 1024 bytes to a connec9on and then a limle later writes another 1024 bytes. The receiver then does a read for 2048 bytes. With a message stream, the receiver will get two messages, of 1024 bytes each. With a byte stream, the message boundaries do not count and the receiver will get the full 2048 bytes as a single unit. The fact that there were originally two dis9nct messages is lost.

4 P5 Nego9a9on has to do with geqng both sides to agree on some parameters or values to be used during the communica9on. Maximum packet size is one example, but there are many others.

5 P6 Both models are based on layered protocols. Both have a network, transport, and applica9on layer. In both models, the transport service can provide a reliable end-to-end byte stream. On the other hand, they differ in several ways. The number of layers is different, the TCP/IP does not have session or presenta9on layers, OSI does not support internetworking, and OSI has both connec9on-oriented and connec9onless service in the network layer.

6 P7 A) Data link layer B) Network layer

7 P8 Among other reasons for using layered protocols, using them leads to breaking up the design problem into smaller, more manageable pieces, and layering means that protocols can be changed without affec9ng higher or lower ones.

8 P9 TCP is connec9on oriented, whereas UDP is a connec9onless service.

9 P10 Connec9on-oriented communica9on has three phases. 1) In the establishment phase a request is made to set up a connec9on. 2) Only azer this phase has been successfully completed can the data transfer phase be started and data transported. 3) Then comes the release phase. Connec8onless communica8on does not have these phases. It just sends the data.

10 P11 Frames encapsulate packets. When a packet arrives at the data link layer, the en9re thing, header, data, and all, is used as the data field of a frame. The en9re packet is put in an envelope (the frame), so to speak (assuming it fits).

11 P12 With n layers and h bytes added per layer, the total number of header bytes per message is hn, so the space wasted on headers is hn. The total message size is M+nh, so the frac9on of bandwidth wasted on headers is hn/(m + hn).

12 P13 If the network tends to lose packets, it is bemer to acknowledge each one separately, so the lost packets can be retransmimed. On the other hand, if the network is highly reliable, sending one acknowledgement at the end of the en9re transfer saves bandwidth in the normal case (but requires the en9re file to be retransmimed if even a single packet is lost).

13 P14 In a NAK only protocol, the loss of packet x is only detected by the receiver when packet x+1 is received. That is, the receivers receives x-1 and then x+1, only when x+1 is received does the receiver realize that x was missed. If there is a long delay between the transmission of x and the transmission of x+1, then it will be a long 9me un9l x can be recovered, under a NAK only protocol. On the other hand, if data is being sent ozen, then recovery under a NAK-only scheme could happen quickly. Moreover, if errors are infrequent, then NAKs are only occasionally sent (when needed), and ACK are never sent a significant reduc9on in feedback in the NAK-only case over the ACK-only case.

14 P15 First of all, reliable communica9on (in our sense, that is, acknowledged) may not be available. For example, Ethernet does not provide reliable communica9on. Packets can occasionally be damaged in transit. It is up to higher protocol levels to deal with this problem. Second, the delays inherent in providing a reliable service may be unacceptable, especially in real- 9me applica9ons such as mul9media. For these reasons, both reliable and unreliable communica9ons coexist.

Programmation système

Programmation système Problems Problem 1 - Message from ATM to Server Message name HELLO {userid} PASSWORD {password} BALANCE WITHDRAWAL {amount} Purpose Let server know that there is a card in the ATM